1. Field of the Invention
The present invention relates to computer graphics systems, and more specifically to a method and apparatus for displaying images on a digital display unit when an analog-to-digital converter (ADC) may be unable to sample an analog display signal at a desired high sampling frequency.
2. Related Art
Digital display units are often used to display images in computer systems. A digital display unit typically receives an analog display signal (e.g., in RGB format) and generates an image encoded in the analog display signal. Digital display units are characterized by discrete points (termed pixels) on a display screen usually contained in each digital display screen. Each point is generally actuated to a varying degree as determined by a received analog display signal. Such actuation generates an image encoded in the received analog display signal.
Analog display signals usually contain multiple frames (hereafter "display signal frames"), with each frame containing multiple horizontal lines. Each display signal frame typically represents an image to be displayed on a digital display screen at a given instance in time. In response to receiving multiple display signal frames, a digital display unit refreshes a display screen with individual images encoded in the received individual display signal frames. Accordingly, a digital display unit may need to display images encoded in the received display signal frames at the same rate as that at which the display signal frames are received. In general, a higher refresh rate (which usually equals the received frame rate) results in an image of a better display quality. Therefore, there is a general need to provide analog display signals at high frame rates.
Digital display units often contain an analog to digital converter (ADC) to sample each display signal frame. The sampled values are commonly used to determine the degree to which each pixel of a digital display screen is to be actuated. A typical ADC needs to sample each horizontal line of a frame a desired number of times. Thus, the desired sampling rate (number of samples per second) of an ADC in a digital display unit is generally computed by multiplying the three factors--the desired number of times each horizontal line has to be sampled, the number of vertical lines in each frame and the number of frames received in each second (frame rate).
For example, in a conventional VGA environment with 800 samples per each horizontal line, 525 vertical lines per frame and with a frame rate of 60 Hz (frames/second), an ADC with a sampling rate of about 25.2 MHZ may be needed. On the other hand, for high resolution monitors with higher frame rates, ADCs having sampling rates as high as 170 MHZ may be needed. In general, the desired sampling frequencies are increasing over time in an attempt to provide better display quality which results from higher frame rates (refresh rates).
Unfortunately, ADCs with higher sampling rates (e.g., 170 MHz) can be expensive, and therefore undesirable in certain situations. In addition, the high sampling rates may not be needed when operating with display signals having lower frame rates. Thus, it may be desirable to provide an ADC with low sampling rate in a digital display unit, but provide a scheme which enables the display of images encoded in display signal frames with high frame rates.
Therefore, what is needed is a method and apparatus for displaying images on a digital display unit when an analog-to-digital converter is unable to sample an analog display signal at a desired high sampling frequency.